Got an iPad yesterday (a little overpriced here in Moscow, 715$ for cheapest model without any cover and such but for me it's worth it anyway) so I'm testing all those apps actually on it, liking the iPad btw, good battery and no need to carry laptop anymore, I still rely on laptop for serious work though
Tried Mobilenoter and my opinion is that it's a tool for people who are already somewhat stuck with onenote because they have all their data there. Mobilenoter allows to view onenote data on the iPad nicely but if you want to actually process and organize things right on the iPad then it's not good because while you can add notes and edit existing notes, the functionality is very limited, you can't even copy paste...
So I'm not going to use it.
I'm not going to use Springpad either because it's lacking in offline features and Evernote is still better overall for General reference material.
So far I've decided to use Evernote for General Reference and use mind maps for Project Support.
I'm using iThoughtsHD for mind maps and I'm just going to store all notes, files(linked with dropbox), images and etc. right in the mind maps without any other tool, so General Reference and Project Support materials won't be linked pretty much at all.
So far it looks like mind maps for Project Support materials is the best tool and gives unlimited flexibility in terms of structure and organization while Evernote looks like a very nice tool to just file all other General Reference in a simple A-Z type tagging system, without any sub-tags.
I also very interested in how other people are using iPad for GTD!

It's so cool to have all your stuff available all the time on such a portable device and also with a very good battery and light weight